Rihanna Reveals How She Got Buff

In the video for her hit "Umbrella," Rihanna shows off her new haircut – and her newly buff bod. But she says she never tried to shed pounds.

"It wasn't so much losing weight as being fit and toned and healthy. I just didn't feel great," the singer, 19, told PEOPLE while rehearsing for her Sunday night performance at the MTV Movie Awards in Los Angeles.

"Now I'm working out and trying to eat healthy. That's making me lose weight without even thinking about it. I'm not intentionally trying to get skinny."

Her fitness regimen includes lots of cardio and weight training two or three times a week. "I work with a trainer," she said. "We work on the arms. I don't want manly, muscular arms. We work out my butt because I love my butt. [But] my legs are my main focus. I'm obsessed."

She's also watching her diet, eating lots of carrots, lettuce, cucumbers and egg whites and snacking on fruit all day, and drinking plenty of water. "I like carbs," she said, "but at the end of the day, carbs are the enemy."

And though she said she likes to flaunt her back and hide her stomach, she hardly looked self-conscious in a Dolce & Gabbana S&M-style corset dress during her performance with Jay-Z at the MTV awards.

Her duet with the rapper was a hit, but there was one other person in the audience whom Rihanna – who says she's planning to take acting lessons – is dying to work with: Sacha Baron Cohen. "He is so funny!" she said. "I could watch Borat every day and not get tired of it."